Flame-resistant reactive resin molded materials which can be processed reliably are required for covering and encapsulating electronic components. Casting technology is the customary form of application for passive components in particular. In addition, reaction injection molding is also used.
As a rule, resin and curing agent components are stored and handled separately from one another and are not combined until immediately before application and processed to give the ready-to-use reactive resin composition. The processing units required for this are cost-and maintenance-intensive, since the ready-to-use reactive resin composition cures completely relatively rapidly and thus has to be removed from the processing units at regular intervals.
Advantages during processing are achieved with one-component reactive resins which, because of the absence of premixing of the components, ensure a higher process reliability and therefore a reproducible quality if the one-component resins can be stored at room temperature for at least three months and if the curing conditions are moderate and compatible with the component to be embedded.
Customary one-component reactive resins are based on epoxy resins and dicyandiamide. They are as a rule employed as plastics. Such systems are unsuitable for casting resin uses because of the incompatibility of dicyandiamide in epoxy resins and the curing temperatures required (&gt;150 C.).
EP-0 408 990 A3 discloses a reactive resin system which cures by means of heat and comprises a polyfunctional epoxide, a phosphorus compound containing epoxide groups, a polyfunctional isocyanate and a curing catalyst, and from which poorly combustible molded materials can be produced.
Another possibility of achieving lower curing temperatures is the storage of systems which are reactive per se at a reduced temperature. For example, ready-to-process and completely worked-up reactive resins are cooled at minus 25 C. and can be stored at this temperature for up to six months. However, this gives rise to considerable outlay on transportation and holding of stocks. Before use, these products must be heated up to the processing temperature with exclusion of moisture.
Reactive resins which can be cured by radiation are as a rule stable to storage. However, they are suitable only for uses in thin layers, which should not be shaded from the irradiation.
Another alternative is constituted by one-component reactive resins which cure completely by means of heat under cationic catalysis. Catalytic activation is effected by irradiation, a suitable photoinitiator dissociating into cations. Further complete curing is effected catalytically and is accelerated by increasing the temperature.
UV activation of such systems can be effected here after application, for example to a substrate, as is known, for example, from EP 0 094 915 A. However, a liquid reactive resin which is stable to storage for weeks is not said to be formed in this manner.
U.S. Pat. No. 4,880,662 proposes a device for activating suitable UV-activatable one-component systems by irradiation before application, so that the reactive resins are also suitable for shaded components, and in particular for embedding components.